fix(policy): a budget declaration must not switch off max_daily_budget_increase_pct

[hyoshi]

Problem

The declaration seam from #414 / #416 replaces the built-in key scan for every channel, including current (_budget_inputs, strategy_gate.py). current_key defaults to None, so for any tool that declares its budget the current budget resolves to None, and this branch of evaluate_guardrails can never fire:

if pct_cap is not None and current is not None and current > 0:

A plugin that declares the expected, documented shape — just daily — therefore has max_daily_budget_increase_pct silently switched off for its tools. Not only on the built-in gate: declarations are registered process-globally at import from the entry points, so a plugin's own gate delegating to StrategyPolicyGate gets the same answer. There is no way for the plugin to opt out.

That is the same silent underenforcement #414 exists to remove, reintroduced through the seam itself — and it lands on precisely the plugins that did the right thing and adopted it.

Declaring current does not work around it. unit is one flag covering every declared key, while mureo's own convention (current_daily_budget, passed by the skills — see _mureo-strategy/SKILL.md) is currency units. A micros tool that declares current divides the baseline by 1e6:

Guardrails(max_daily_budget_increase_pct=20)
{"daily_budget_micros": 15_000_000_000, "current_daily_budget": 10_000}   # ¥10,000 → ¥15,000, +50%

daily only        → allowed=True                                    # cap silently dead
daily + current   → "raises spend 149999900% (0 → 15,000)"          # baseline destroyed

Fix

current falls back to the built-in scan unless the plugin declares it.

The distinction is principled, not a special case: daily / lifetime are budgets the tool proposes, so the plugin's vocabulary is authoritative and replacing the scan wholesale is right (an unrelated field spelled amount must not false-trip a cap). The current budget is not something the tool carries at all — it is context the caller supplies, under mureo's own cross-provider convention. A plugin that genuinely does take the current budget as an argument can still declare current, and that declaration still wins.

Tests

TDD, red first, in tests/test_plugin_budget_declaration.py:

• test_undeclared_current_falls_back_to_the_built_in_key — a micros daily declaration + current_daily_budget → the +50% raise is refused (was allowed).
• test_the_fallback_current_is_currency_not_micros — the reason reads 10,000 → 15,000, not 0 → 15,000.
• test_a_declared_current_key_still_wins — an explicit current declaration is not overridden by the fallback.

63 passed across test_plugin_budget_declaration.py + test_strategy_gate.py; ruff / black / mypy clean.

docs/plugin-authoring.md and the BudgetDeclaration docstring updated: they both stated the "replaces every channel" rule that this changes.

Found by

Adopting the seam in a provider plugin: declaring daily made the plugin's own percentage-cap regression test start passing calls it used to refuse.